1. Field of the Invention
The present invention relates to an optical fiber sensor which includes a light transmitter/receiver portion, an optical sensor head portion remote from the transmitter/receiver portion, and an optical fiber or fibers for optically coupling the transmitter/receiver portion and the sensor head portion.
2. Discussion of the Prior Art
For effecting precise measurement of physical quantities such as length, angle and pressure, it is proposed to use an optical measuring system or apparatus which includes a light transmitter/receiver portion having a light source and a light detector, and a sensor head portion optically coupled with the transmitter/receiver portion. The light source produces a reference beam and a measuring beam, and the sensor head portion is adapted to cause a transmission parameter of at least the measuring beam to be influenced by a change of an external subject or condition. The influenced measuring beam and the influenced or non-influenced reference beam are returned from the sensor head portion to the light detector of the transmitter/receiver portion, so that the change of the external subject is detected based on the received measuring and reference beams.
An example of such an optical measuring system is illustrated in FIG. 10, wherein a He-Ne laser source 70 of frequency-stabilized type produces a first and a second linearly polarized beams, under the control of a laser tuning circuit 69. The first and second linearly polarized beams have different frequencies f1, f2 and mutually perpendicular polarization planes. Each of these beams is split by a reference beam splitter 71 into two components. One of the two components which is reflected by the beam splitter 71 is split by a beam splitter 66 into two halves, one of which is received by a feedback photosensor 67, and the other of which is received by a reference photosensor 72 via a mirror 68. The other component of each of the first and second linearly polarized beams which is transmitted through the reference beam splitter 71 is split by a polarizing beam splitter 73, into a reference beam having the frequency f2 and a measuring beam having the frequency f1. The reference beam is reflected by a stationary mirror 74, while the measuring beam is reflected by a movable mirror 75. These reflected reference and measuring beams are combined with each other by the polarizing beam splitter 73, and the composite beam is received by a first photosensor 76. This first photosensor 76 detects a beat frequency of the received reference and measuring beams. Since the frequency of the received measuring beam was shifted or changed by .DELTA.f1 due to Doppler effect by a movement of the movable mirror 75, the beat frequency detected by the first photosensor 76 is equal to f2-(f1.+-..DELTA.F1). On the other hand, the beat frequency detected by the reference photosensor 72 is equal to f2-f1. The outputs of the photosensors 72, 76 are applied to an output device 77 which incorporates a pulse converter which produces a pulse each time the movable mirror 75 is moved by an amount corresponding to .lambda./2. The output device 77 further incorporates a counter for counting the number of the pulses produced by the pulse converter, and thereby accurately calculating a difference .DELTA.f1 between the beat frequencies detected by the photosensors 72, 76. In this arrangement, the polarizing beam splitter 73, fixed mirror 74 and movable mirror 75 constitute a sensor head portion which is remote from a light transmitter/receiver portion, which includes the other components illustrated in FIG. 10.
In the optical measuring system as described above, however, the light beams are transmitted or propagated through the atmosphere between the light transmitter/receiver portion and the sensor head portion. Therefore, a space through which the optical paths extend should be free from any obstacle disturbing the optical paths. Further, the beams propagated through the optical paths are influenced by streams of the atmosphere, whereby the measuring accuracy is lowered. The use of suitable covering means for surrounding and protecting the optical paths inherently results in increased size and cost of manufacture of the measuring system.